This invention relates generally to an x-ray damage shield and a method of manufacturing the shield. More particularly, it relates to a self aligning x-ray damage shield for protecting an inter-scintillator reflector and a method of manufacturing such a shield.
Solid state detectors for computed tomography (CT) imaging use scintillators to convert x-rays into scintillation light which itself is converted to an electrical signal with a photodiode. Detector arrays are typically comprised of scintillator pixels separated by a reflecting material used to pipe the scintillation light towards the diode. Scintillator thicknesses and pixel widths required by such detectors result in aspect ratios (the ratio of the height to width of a pixel) such that on average, the scintillation light reflects off the reflecting material several times before exiting to the diode. For this reason, the materials useful as a reflector are limited to materials that are highly reflecting at the scintillation light wavelengths emitted by the scintillator.
Appropriate reflector materials include high refractive index solid materials such as TiO.sub.2 formed in a castable low index medium such as an epoxy. One drawback of such a system is the darkening of the epoxy matrix when it is struck by a dose of x-rays commonly used in CT imaging. A typical dose over the life of the detector is 1 Mrad. This darkening results in lower reflectivity and less efficient collection of the scintillation light, and thus a lowering of the sensitivity of the x-ray detector.
Furthermore, the darkening is often not uniform over the entrance face of the detector. This lack of uniformity in darkening can result in image degradation if the detector is not properly calibrated. In addition to the reflector material itself, the diode below the reflector is also sensitive to radiation and must be protected from the x-ray beam.
Current CT detectors use a collimator assembly to protect the reflector epoxy material from damage by x-rays. This assembly consists of tall tungsten plates aligned perpendicular to the plane of the x-ray fan beam. This assembly is primarily used to minimize scattered x-rays from reaching the scintillator, but is also used to protect the reflector material between pixels from the x-rays. For multi-slice CT, where the detector is segmented in the direction parallel to the fan beam, wires are used to protect the reflector and diodes. These wires are strung between the deep plates in grooves machined in the plates.
The manufacturing of such a two dimensional collimator with plates and wires is complex. The separate construction of the collimator with protective wires and the scintillator/reflector body requires accurate alignment of these devices during construction of the complete detector. This alignment cannot be done optically since the reflector material between the scintillator pixels ("interscintillator reflector") is obscured by reflector material covering the top of the pixels ("surface reflector"). Therefore, either x-ray alignment or rigorous dimensional tolerances must be used to ensure that the reflector material is aligned with the protective wires.